Varistors

ABSTRACT

A plurality of heat radiating axial ribs are provided on the surface of a varistor. For a varistor in the form of a hollow cylinder, such ribs are formed on both or either one of the inner and outer surfaces of the varistor.

C United States Patent 11 1 1111 3,753,198 Shimizu Aug. 14, 1973VARISTORS [56] References Cited [75] Inventor: Mano Shlmlzu,Suginami-ku, UNITED STATES PATENTS Tokyo, Japan 1,509,495 9/1924 Slepian3,380,936 4/1968 Masuyama et a] [73] Dam onky united 2,358,406 9/1944Lichtgam Japan 686,245 11/1901 Bachmann et al. 573,558 12/1896 Voss [22]1970 3,243,753 3/1966 K0111 338/31 [21] Appl. No.: 73,390

Primary Examiner-C. L. Albritton [30] Foreign Appuuflon priorityAttorney-Chittick, Pfund, Birch, Samuels & Gauthier Sept. 19, 1969 Japan44/88857 Sept. 19, 1969 Japan 44/88858 ABSTRACT A plurality of heatradiating axial ribs are provided on [52] US. Cl. 338/20 the surface ofa varistor. [5 III. For a van'stor in the f f hollow y i Such ribs [58]Field of Search 338/20 are formed on both or either one of the inner andouter 338/31, 329, 332, 333; 219/505, 538, 540, surfaces ofthe mister 2Claims, 8 Drawing Figures 0-. a 2 r --.Q s Lu .LENGTH OF THE VARISTORPatented Aug. 14, 1973 3,753,198

TEMP.

LENGTH OF THE VAR! STOR IN VE N TOR M sAO SHIMIZU BY WYB-ATRLQ..

ATTORNEY v VARISTORS BACKGROUND OF THE INVENTION This invention relatesto a varistor and more particularly to a high voltage high capacityvaristor of improved heat dissipating construction.

As is well known in the art, varistors are advantageous in that they canwithstand lar'ge overloads, are not destroyed by an extremely high surgevoltage, and

the temperature coefficient of the resistor component is relatively lownear normal temperature. However, in high voltage high capacityvaristors as the current and hence the quantity of heat generated bythecurrent are substantial it is difficult to efficiently dissipate theheat, due mainly to the construction of the varistors. Consequently, thetemperature coefficient of the varistors increases in the negativedirection, thus decreasing the internal resistance of the varistor. Forexample, in a rod shaped varistor applied with a voltage of about 3OKVand passing a current of about lmA, especially with those having a smallcross-sectional area, the quantity of the heat generated is very largewith the result that the temperature distribution along the longitudinalaxis of the varistor is such that the temperature is the high est at theaxial center and gradually decreases toward the opposite ends. In otherwords, the temperature rise of the varistor results in a decrease in theavalanche voltage, thus deteriorating the characteristics of thevaristor.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide an improved varistor having heat radiating means forefficiently dissipating the heat to prevent temperature rise.

Another object of this invention is to provide an improved varistorhaving heat dissipating means to make substantially flat the temperaturedistribution along the varistor.

Still another object of this invention is to provide a varistor havingefficient heat dissipating means capable of preventing decrease in theavalanche voltage due to temperature rise.

According to one feature these objects can be accomplished by providingheat radiating means in the form of a plurality of axial ribs on theperipheral surface of a solid varistor. In accordance with anotherfeature of this invention the varistor is made hollow to substantiallyincrease the heat radiating surface area. Further, a plurality of axialribs are provided on the outer and or inner peripheries of a hollowcylindrical varistor over a limited axial length thereof to locallyincrease the heat radiating area thus obtaining a uniform temperaturedistribution.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can be more fullyunderstood from the following detailed description when taken inconjunction with the accompanying drawings in which:

F 16.1 is a perspective view of one embodiment of the novel varistor;

F102 is a cross-sectional view taken along a line 2'2 in FIG];

FIG.3 is a perspective view of a modified embodiment of this invention;

FIG.4 is a cross-sectional view taken along a line 4-4 in FIG.3;

FIGS is a cross-sectional view similar to FIG.4 of another embodiment ofthis invention;

FIG.6 shows still further modification of this invention;

. FIG.7 is a sectional view taken along a line 7-7 in FIG.8 is a graphtov show temperature distribution characteristics along the length ofthe varistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to theaccompanying drawing, the varistor shown in FIGS. 1 and 2 and generallydesignated by a reference numeral 10 comprises a fired solid main body12 in the form of a solid round rod of small diameter and terminal metalcaps 14 and 16 secured to the opposite ends of the main body 12. Leadwires 18 and 20 are secured to the end walls of terminal caps bysoldering or welding, for example. A plurality of radial ribs 22 areformed on the outer periphery of the main body 12 along the entirelength thereof. These ribs can be formed by providing correspondinggrooves for the extrusion die or molding die used for shaping the mainbody.

As above described since the temperature rise of the varistor is mostsignificant at the axial center, these ribs 22 may be formed only on theaxial central portion of the main body 12. In such a case, to improvethe-heat dissipating efficiency, thenumber of ribs may be increased.Alternatively, the radial height of the ribs may be increased toincrease the heat dissipating area of the central portion of thevaristor. With this construction the heat radiating surface area of themain body 12 is increased locally or as a whole thus improving theefficiency of heat radiation. For this reason, the temperature rise ofthe varistor is decreased, and the temperature distribution along theaxial length becomes flat. Accordingly, it is possible to preventdecrease of the avalanche voltage of the varistor thus assuring normaloperation thereof at a predetermined voltage in a circuit in which thevaristor is connected.

FIGS. 3 and 4 shows a modified embodiment of this invention in which themain body 24 has a flat outer surface and a central bore axiallyextending through the main body. Terminals 36 and 38 in the form ofmetal bands are secured by screws (not shown) to the opposite ends ofthe main body 24. In this embodiment the heat generated in the varistoris dissipated from its inner and outer surfaces to increase the heatdissipation, so that it is possible to limit the temperature rise belowa permissible value even when the current is increased to some extent.Increase in the sectional secondarymoment increases the mechanicalstrength of the varistor against mechanical shocks and vibrations.

In another embodiment, the heat dissipating surface area of the varistorshown in FIGS. 3 and 4 is increased further by providing a plurality ofaxial ribs 28 on the inner surface as shown in FIGS. Again such ribs maybe formed at the central portion alone to make uniform the temperaturedistribution.

In yet another modification shown in FIGS.6 and 7, the hollowcylindrical main body 24 of the varistor shown in FIG.3 is provided witha plurality of axial ribs 40.

FIG.8 compares temperature distribution curve a of a prior art varistorand those b and c of varistors embodying this invention. As can be notedfrom FIG.8,

curve b showing the temperature distribution characteristic of a solidvaristor with ribs on the outer periphery (FIG.1) or a hollowcylindrical varistor (FlG.3) is lower than curve a and the provision ofribs only in the central portion of the varistor flattens thetemperature distribution curve as shown by curve 0. From this it will beclear that this invention provides a varistor of improved heatdissipation. In other words, even when larger current is passed it ispossible to prevent excessive temperature rise. The metal terminals andlead wires connected thereto further enhance the heat dissipationwhereby the current carrying capacity of the varistor is improvedwithout the risk of decreasing the avalanche voltage.

What is claimed is l. A varistor comprising a solid rod shaped mainbody, terminals secured to the opposite ends of said main body and aplurality of heat radiating axial ribs formed on the surface of saidmain body at only the axial center portion thereof so that thetemperature axially along said main body is substantially uniform whencurrent is applied to said terminals.

2. A varistor comprising a solid rod shaped main body, terminals securedto the opposite ends of said main body and a plurality of heat radiatingaxial ribs formed on substantially all of the peripheral surface of saidmain body, those ribs located at the axial center portion of said mainbody having a greater height so that the temperature axially along saidmain bcldy is substantially uniform when current is applied to saidterminals.

I. I i 1.!

1. A varistor comprising a solid rod shaped main body, terminals securedto the opposite ends of said main body and a plurality of heat radiatingaxial ribs formed on the surface of said main body at only the axialcenter portion thereof so that the temperature axially along said mainbody is substantially uniform when current is applied to said terminals.2. A varistor comprising a solid rod shaped main body, terminals securedto the opposite ends of said main body and a plurality of heat radiatingaxial ribs formed on substantially all of the peripheral surface of saidmain body, those ribs located at the axial center portion of said mainbody having a greater height so that the temperature axially along saidmain body is substantially uniform when current is applied to saidterminals.